[Chaos]

Lolzy has a point: Lowering the damage dealt doesn't change the fact that a tank can still abuse it to ruin someone's day. That doesn't fix the problem.

If you want to quickly throw on a solution that will actually dissuade people from using it as a go-to tank skill, then give it a Once Per Battle stipulation, or force the user to have X% or less HP before they can fire it off.